Doc Brown has created a new time machine that brings historical figures to him. Bringing the past to the present seems to be less dangerous than transporting himself to the past. He is sharing his invention with the citizens of Hill Valley when something goes wrong.
BACK TO THE ENLIGHTENMENT
Marty, my boy, I've done it! I've made a major breakthrough, I'll need your assistance.
The last time you said that, I almost ended my life before it began.
This time it will work, and you won't have to go anywhere. The past will come to us! In a few moments, some of the greatest minds of the Enlightenment will walk through that door.
This dusty hovel is no place for a Czarina. An Enlightened Despot such as Catherine the Great should not be in such conditions as these.
Je ne sais pas oú suis! I can figure this out using deductive reasoning. I am René Descartes, after all. "Cogito ergo sum". I am still thinking, therefore, I am still alive.
With equipment like this, I can disprove the Ptolemiac system before the Church finds me.
Already done, man. Already done.
Great Scott!! Catherine the Great, Descartes, Copernicus, and Galileo are in my lab!
I have the foundation for the greatest intellectual salon ever!
Well, Doc, after you get your hair done, I'm going to borrow the DeLorean and take these guys home.